The present invention relates to bioelectrical measurement, particularly to a method for localizing electrical activity in the body, such as in the brain or heart.
As is well known, the body produces electrical fields and waves, and these provide important information regarding body function. To acquire this information, it is often necessary and usually desirable to determine the sources of electrical energy inside the body whose electrical effects are measured at the body surface. Conventional source analysis begins by measuring the electrical potential at the surface of the body proximate an underlying organ of interest. For analyzing sources in the heart, an electrocardiograph (xe2x80x9cECGxe2x80x9d) measures the electrical potential on the torso, while for analyzing sources in the brain, an electroencephalograph (xe2x80x9cEEGxe2x80x9d) measures the electrical potential on the head.
To localize the sources in the organ or tissue that are responsible for given measured surface potentials, a computer-based model is made of the organ or tissue in terms of the conductive paths leading therefrom to the body surface. Ideal sources, such as single or multiple dipoles or extended dipolar sheets, are modeled in the computer and manipulated within a model of the organ or tissue while surface potentials are calculated until satisfactory agreement is reached between the calculated values of potential and those actually measured on the body. Such models require specification of both the body geometry and the body impedance or conductivity as a function of position within the body.
In conventional EEG models, the head is represented by a small number of spherically concentric tissue layers or regions, typically divided as brain, cerebrospinal fluid, skull and scalp, simplifying the model so as to require only four impedance values, one value for each region. In conventional ECG models, the torso is similarly considered to consist of the heart, lung, body cavity and skeletal muscle. Such models make use of impedance values that have been estimated by measuring laboratory subjects.
The advent of magnetic resonance imaging techniques now provides for more geometrically detailed head and torso models, and consequently more precise source localization should be possible. Accordingly, the art has begun to focus on obtaining more precise measurements of impedance of bodily tissue for use in the published tables. However, the increased precision has not translated to the expected degree of increased accuracy. The reason for this has not heretofore been understood.
Accordingly, there is a need for a method for localizing electrical activity in the body that provides for further reducing source localization errors. The ability to accurately localize a source of electrical activity in the body may also be used to advantage in stimulating that source.
The method for localizing electrical activity in the body of the present invention solves the aforementioned problems and meets the aforementioned needs by providing a plurality of electrical devices on a carrier which distributes the devices over and applies the devices to a selected portion of the surface of the body. The carrier is preferably the geodesic net described in the inventor""s U.S. Pat. No. 5,291,888. Each of the devices is adapted to sense voltage (or current), and to apply current (or voltage) at substantially the same location on the body. One of the devices forms a first port with a reference device for applying an input current (or voltage) to the body. Another of the devices forms a second port with the reference device for sensing the resulting voltage (or current).
With a number of current (or voltage) inputs and voltage (or current) outputs distributed over the surface of the body, a best-fit computer model is employed to estimate the conductivity of the underlying tissue as a function of position. Measured potentials of the electrical activity within the body, i.e., without applying current (or voltage) by the devices, are employed to localize the sources of that activity within the body in a standard manner, using the aforedescribed estimates of body conductivity.
Measured potentials for source localization are taken at substantially the same locations as measured potentials for impedance characterization. In some applications, the potentials for both purposes are also taken at substantially the same time.
Magnetic resonance imaging may be used in conjunction with electrical inputs to the surface of the body to develop a conductivity model for underlying body tissue, by taking advantage of the fact that current flow in the body tissue affects magnetic resonance signals in a known way. Other imaging technologies may also be used for this purpose.
Conductivity models obtained according to the invention may also be used with source localization to determine how to apply surface potentials for stimulating function in underlying body tissue at a precise location.
Therefore, it is a principal object of the present invention to provide a novel and improved method for localizing electrical activity in the body.
It is another object of the present invention to provide such a method that further reduces source localization errors.
It is yet another object of the present invention to provide a method that provides for physiological stimulation of body tissue at a precise location in the body.
It is still another object of the present invention to provide a method for determining the conductivity of internal body tissue, for use in source localization and physiological stimulation.
The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.